Prof. Leonhardt

The science of invisibility could make micro machines work without friction. Image © TEDxBrussels/Scorpix.

From the galactic to the quantum, the science of invisibility is revealing new ways to manipulate the world, said Professor Ulf Leonhardt, from Israel’s Weizmann Institute, after giving a presentation at TEDxBrussels on 1 December.

You discussed the science of invisibility during your presentation, and in your European Research Council (ERC)-funded research you are linking theoretical physics with invisibility, but what does that mean in practice?

‘Invisibly is only a small part of my research and it’s one starting point. What I use in my research is the fact that transparent materials like glass or water appear to change the perception of space. What I am doing is applying this analogy between optical materials and geometry to other areas of research.’

Which areas does this relate to?

‘We are running an experiment to make analogues of the event horizon of a black hole. We are making black holes in the laboratory using tricks from optics. These black holes are close enough that we can draw some conclusions from them and make measurements that are meaningful.

‘I’m also doing research on so-called Casimir forces. These are forces between electrically neutral substances like a piece of paper and glass. They are important for micro and nano machines because, on very small scales, these forces become strong and they lead to friction and stiction, so parts of the machinery sticks together, which is not really what you want.

‘What I investigate is the idea of turning this around, so changing the sticky Casimir force into a repulsive force for making frictionless micro-machinery. The tricks for this come from the fact that optical materials transform distances. If you have a force that acts on a very small distance, then it would be an advantage to increase that distance, or to turn it from a positive to a negative one.

‘This really is the primary direction of my research. I use the science of invisibility for various practical applications.’

Why doesn’t paint stick to the inside of a bucket?

Dutch physicist Hendrik Casimir was asked this very question in the 1940s by his employer, the company Phillips. His answer, formulated in 1948, was that there was a new force at work between molecules, which has now been called the Casimir force.

The Casimir force is believed to cause uncharged materials such as paper and glass to attract or repulse each other by borrowing light. Under Casimir’s theory, the paper would borrow light from the quantum realm, polarising the molecules in the glass, which returns the charge to the paper. The paper then returns the light to the quantum realm.

This means that, at small distances, these materials can behave as if there is electromagnetism passing between them, even though no energy is required. Therefore the walls of the bucket can repel the paint, even though they have no charge.

Can you explain the science of invisibility in simple words?

‘Imagine you are looking into a fish tank. You see a fish inside the tank, and then you look at it from different angles, you see a dramatic change in the position of the fish. The water and the glass walls of the aquarium change the perception of space.

‘This is exactly the same in astrophysics. You have a change in the geometry of space and time caused by gravity. The analogy between optical materials and the physics of space is the key idea behind my research, and then you ask what consequences does this idea have. What things can I do with it?

‘Using the analogy of the aquarium, you could design a very unusual aquarium that has the property that it de-magnifies the objects inside. If you focus on one fish it becomes very, very small up to the size of a single point. A single point you cannot see, and so it has disappeared. You get a cloaking device.

‘We are making black holes in the laboratory using tricks from optics.’

Professor Ulf Leonhardt, Weizmann Institute, Israel

‘You could also think about creating an event horizon. Imagine a river that flows faster and faster towards a waterfall. Some poor fish – salmon, for example – struggle to swim upstream. However, beyond a certain point the river gets faster than the fish can possibly swim. Beyond this point the fish are trapped and doomed to drift towards the waterfall. This point is like the event horizon of a black hole. In space, the role of water is played by gravity, you can understand the gravitational field as a fluid flowing towards the centre of attraction which takes everything with it, including light. If that flow becomes faster than the speed of light, you have a horizon, and this is exactly what is happening at the event horizon of a black hole.’

That’s fascinating. The event horizon that you make with running water, what have you learnt from that?

‘Of course I should say we don’t do this with water, we do this with light pulses in fibre optics. To create a medium that is as fast as light you’d better use light as a medium itself. In our experiments, the light travels as a pulse in a glass fibre. Suppose another light beam tries to chase the pulse. When it gets close to it, the pulse slows it down; it cannot get past the pulse. The pulse makes a horizon for light. From this analogy we have learnt that effects such as Stephen Hawking’s famous prediction that black holes radiate may also happen in fairly ordinary materials such as in fibre optics. They are not restricted to astrophysics but they belong to a much wider class of phenomena that happen in the lab, or in other scenarios.’

Do your findings have any bearing on the popular understanding of black holes and event horizons?

‘I think what they do is demystify black holes. Their main character can be understood in an ordinary way. You can in fact understand them with everyday logic. There comes of course a point where you realise how special black holes are, but to a large extent it is possible to understand them with analogies like flowing water or waves propagating in moving fluids. You can do this and you can get very far with this.’

What’s the next step, what do you hope that this will result in?

‘What we hope is to detect Hawking radiation in the laboratory. We have almost everything ready for this. Then, on the Casimir front, we are also making progress. There has been a long-term problem with attractive and repulse Casimir forces – they should depend on the shape of things. A hollow sphere, for example, should have a repulsing Casimir force – it would explode if it is too small. We will check this prediction, and then we can go on and see what we find.’

When do you hope to show that?

‘I’m not going to make an exact prediction on how long it will take for us to detect Hawking radiation. If things work out well, we will have that within a year. If there are complications, I don’t know how long this will take. So we will see. But, hopefully, within the period funded by the ERC grant we should have some good results. And then, on the Casimir forces, also within the year we will have something interesting, I am sure.’

What do you plan to do after that?

‘It depends on what we find. The nature of research is that it is unpredictable. It is only interesting if it is surprising and therefore the best research is unpredictable. The more interesting the findings, the more unexpected they are. I can only predict whether my subject is a fertile area. If it leads to good questions then there will be interesting answers.’

Is there anything that you would like to add?

‘There are different types of research, there is problem-orientated research and there is basic research. Problem-oriented research begins with a specific problem you want to solve. You throw ideas and money at it and eventually the problem is solved – if it can be solved. Basic research begins with an idea, like the connection between geometry and media, and then you develop this idea and you find the problems you can apply it to. Basic research is linked to the unknown, you may find things that you have not expected, and therefore in the end it might be a very fruitful avenue to go down. Most research is problem-oriented research, and that is perfectly fine, but the really important breakthroughs come from basic research. What the ERC funds often is basic research, and this is important and very valuable.’

Prof. Leonhardt's full presentation at TEDxBrussels.